Currently, digital broadcasting systems are becoming increasingly popular for delivering high quality audio and video content to individual consumers. In a typical digital broadcasting system, as depicted in FIG. 1, an analog signal on an input line 10 is sampled by a digital encoder 120. The samples are digitized and encoded to form data frames which contain digital representations of the analog signal. The data frames are then combined to form a master data frame defining a string of bits known as a bit stream on line 130. The master data frame is then modulated by a digital modulator 140 for broadcasting the master data frame over a broadcasting channel 150. The modulated signal is then received and demodulated by a digital demodulator 160 to derive a bit stream on line 170 which is equivalent to the original bit stream on line 130. A digital decoder 180 then decodes the bit stream 170 to obtain an analog signal at output line 190 which can be output to an analog device such as a speaker.
The digital encoder samples the analog signal, digitizes the samples, and encodes the samples. Generally, the digital encoder collects a number N of digitized samples and generates a frame of encoded data based on the set of N digitized samples. FIG. 2 depicts a typical digital encoder. An analog signal on input line 210 is sampled by the digital encoder 220 at its input for a specified period of time, x. The digital encoder 220 then produces an output signal on output line 230 which is a digital representation of the analog signal on input line 210. In a fixed rate encoder, the output for each set of N digitized samples of the analog signal on input line 210 will contain the same number of bits which are defined by individual data frames of equal length.
FIG. 3 depicts a master data frame MF which is completely filled with the individual data frames, m1-mN. Each master data frame MF contains a few bits of master frame information MFi, such as the length of the master frame and the number of individual data frames within the master frame MF. The remainder of the master data frame MF is filled with the individual data frames, m1-mN. The individual data frames, M1-mN, also contain a few bits of individual data frame information, m1i-mNi, such as the length of the frame. Because the individual data frames, m1-mN, are fixed in length, a system can be designed where master data frames MF are entirely filled with a specified number of individual data frames, m1-mN.
Presently, however, many digital encoders are variable rate digital encoders. For example, the digital encoders used to encode audio signals are generally variable rate encoders. In a variable rate encoder, the digital output for each set of N digitized samples of the analog signal may contain a different number of bits which are defined by data frames of unequal length. Variable rate audio encoders are used because they produce better digitized audio signals than fixed rate audio encoders in terms of lowering the number of bits required to accurately encode the audio signal without reducing the quality of the audio signal. The variable rate audio encoders are typically designed around the human perception of audio signals in order to minimize the number of bits required to accurately portray the audio signal digitally.
The audio encoders encode the audio samples into individual frames using a variable bit rate, resulting in individual frames which fluctuate in length based on the complexity of the audio segments being sampled. The sampling of an audio signal having segments, S1-SN, of varying complexity is depicted in FIG. 4. Each segment of the analog audio signal is for a specified period of time, t, and produces frames, m1-mN, which define a number of bits that may vary in number from segment to segment.
Most digital transmission systems transmit digital signals using fixed length master frames, with each fixed length master frames comprising master frame information and a number of smaller frames of encoded data where each frame is preceded by a synchronization pattern. Ideally, the total number of bits defined by all of the digitized samples and synchronization patterns and the master frame information would equal the number of bits defined by the fixed length master frame MF. However, due to the variable bit rate of a variable rate encoder, the total number of bits defined by the encoded audio segments and master frame information may be less than the number of bits defined by the fixed length master frame MF. When the total number of bits is less than the fixed length master frame MF, some of the bits in the fixed length master frame MF are unused. As depicted in FIG. 5, the unused bits in a fixed length master frame MF are generally filled with a fixed pattern, such as all zeros, to indicate that no data is being transmitted. The unused portion may also be used to send other information such as text messages or other data of a non-random nature. However, filling the unused portion with a non-random pattern such as zeros or text, decreases the randomness of the fixed length master frame MF which results in system inefficiency. As is known in the art, system efficiency is maximized in a system which transmits modulated signals by maximizing the randomness of the signal to be transmitted. Non-random data patterns result in a modulated signal with higher peak power. This causes an increase in the systems peak-to-average power ratio (PAR). The higher PAR necessitates more amplifier power to transmit the modulated signal with the increased peaks operating in a linear region of the amplifier. Increasing the randomness of the data pattern results in a modulated signal with a lower PAR, thus reducing the need for more amplifier power. Hence, system efficiency increases as randomness increases.